三維繪圖程式設計 3D Graphics Programming Design 第六章 Bitmap 影像及圖檔格式設定嘉大資工系盧天麒

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1 三維繪圖程式設計 3D Graphics Programming Design 第六章 Bitmap 影像及圖檔格式設定嘉大資工系盧天麒 1

2 In this chapter, you will learn How to use OpenGL bitmaps OpenGL pixel functions How to load and save the Targa image format 2

3 The OpenGL Bitmap Bitmap A rectangular array of pixels One bit of information (a 0 or 1) To be stored about each pixel Example 16 x 16 bitmaps Drawn by OpenGL as the letter X 3

4 The OpenGL Bitmap Draw the bitmap At the lower-left corner Working its way up, row by row The bottom of the bitmap is the first set of data The top of the bitmap is the last set of data 4

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6 Positioning the Bitmap The glrasterpos( ) function Specify the current raster coordinates for drawing bitmaps Define the bottom-left corner of the bitmap s rectangle Set the current raster coordinates to (30, 10) glrasterpos2i(30, 10) 6

7 Positioning the Bitmap You can use glortho( ) or gluortho2d( ) function to define a 2D viewport with orthographic projection For error checking A valid raster position To obtain the current raster position 7

8 Drawing the Bitmap After you set the current raster position You can draw your bitmap with the glbitmap( ) function 8

9 Drawing the Bitmap Draw your bitmap with glbitmap() function void glbitmap(glsizei width, GLsizei height, GLfloat xorigin, GLfloat yorigin, GLfloat xincream, GLfloat yincream, const GLubyte Xbitmap); width and height in pixels at the coordinates(xorigin, yorigin) relative to the current raster position. The values xincream and yincream specify step increments that are added to the current raster position after the bitmap is drawn. 9

10 An OpenGL Bitmap Example Display 50 16x16 bitmaps in random locations during each frame 10

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13 Using images Developers use images instead of the OpenGL bitmap An image might hold anywhere from 8 bits of information per pixel to 32 bits per pixel With OpenGL you can manipulate images pixel by pixel Pixel maps or pixmaps 13

14 Drawing image data Assuming you already have your image data loaded into memory You use the OpenGL function gldrawpixels( ) to display the image data 14

15 Drawing image data The gldrawpixels( ) function looks like this: void gldrawpixels(glsizei width, GLsizei height, GLenum format, GLenum type, const GLvoid *pixels); 15

16 Drawing image data 16

17 Drawing image data 17

18 Drawing image data Here is some code that uses the gldrawpixels( ) function to draw an RGB image of a width and height imagewidth and imageheight unsigned char *imagedata; int imagewidth, imageheight; glrasterpos2i(300, 300); gldrawpixels(imagewidth, imageheight, GL_RGB, GL_UNSIGNED_BYTE, imagedata); 18

19 Reading from the screen You want to read the pixels already on the screen You can save them to disk as an image file OpenGL allows you to do this by providing you with the glreadpixels( ) function void glreadpixels(glint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels); 19

20 Reading from the screen The (x, y) coordinate specifies the lowerleft corner of the rectangle with dimensions defined by width and height To be read from the screen and stored in the pixels parameter The format and type parameters work the same way as gldrawpixels( ) 20

21 Reading from the screen As an example, if you want to read the top half of your OpenGL window into an RGB buffer, you might use the glreadpixels( ) function like this void *imagedata; int screenwidth, screenheight; glreadpixels(0, screenheight/2, screenwidth, screenhieght/2, GL_RGB, GL_UNSIGNED_BYTE, imagedata); 21

22 Magnification, reduction, and flipping OpenGL lets you enlarge, reduce, and flip images with the glpixelzoom( ) function void glpixelzoom(glfloat xzoom, GLfloat yzoom); By default, the xzoom and yzoom parameters are 1.0, meaning the pixel zoom is set to normal viewing mode 22

23 Magnification, reduction, and flipping When you specify negative values, the image is reflected about the current raster position Here are some examples glpixelzoom(-1.0f, -1.0f); //flip image horizontally and vertically glpixelzoom(0.5f, 0.5f); //reduce image to half its original size glpixelzoom(5.0f, 5.0f); //magnify the image 5 times in all directions 23

24 Targa Image Files Targa image format Simple to work with Incorporate transparency Other cool special effects 24

25 The Targa File Format Two parts The header and the data 25

26 Loading Targa Files The CTargaImage class provides mechanisms for loading the Targa image, flipping it vertically, and converting the image data from one format to another 26

27 Loading Targa Files class CTargaImage { private: unsigned char m_colordepth; unsigned char m_imagedatatype; unsigned char m_imagedataformat; unsigned char *m_pimagedata; unsigned short m_width; unsigned short m_height; unsigned long m_imagesize; // swap the red and blue components in the image data void SwapRedBlue(); public: CTargaImage(); virtual ~CTargaImage(); // loading and unloading bool Load(const char *filename); void Release(); // flips image vertically bool FlipVertical(); unsigned short GetWidth() { return m_width; } unsigned short GetHeight() { return m_height; } unsigned char GetImageFormat() { return m_imagedataformat; } // converts RGB format to RGBA format and vice versa bool ConvertRGBAToRGB(); bool ConvertRGBToRGBA(unsigned char alphavalue); }; // returns the current image data unsigned char *GetImage() { return m_pimagedata; } 27

28 Loading Targa Files An example - LoadTGA Load 24-bit and 32-bit compressed and uncompressed Targa images 28

29 Summary You have learned about How to set the current OpenGL raster position and draw the bitmaps To draw the images, read image data from the screen, and perform magnification, reduction, and flipping on image data. How to load and draw Targa images 29

30 References Dave Astle and Kevin Hawkins, Beginning OpenGL Game Programming, 1st Edition, Course Technology PTR,